The invention relates to systems and methods for implementing large CDMA cell sizes, particularly for application in systems which, due to a search window size which limits searching capabilities, can only establish wireless links within cells of a fixed maximum size.
CDMA (code division multiple access) communications systems are currently designed to accommodate a maximum cell size radius of about 62.5 km. For certain applications, such as in large service areas and in coastal service areas, it would be advantageous to be able to provision for larger cell sizes, and to be able to establish traffic channels between mobile stations and base stations within such larger cells.
In order to establish a session in a large cell or any cell for that matter, a mobile station must first xe2x80x9cacquirexe2x80x9d a base station. Existing approaches to base station acquisition are still applicable in large cells. Mobile stations search for base stations within a CDMA communications system by looking for an identification code, such as the IS-95 PN (pseudo-random number) short codes (in-phase code and quadrature code), transmitted by every base station, different sectors of different base stations sending the same short code with a different time shift, known as a PN offset, which provides a unique base station sector identifier. After finding a transmitted PN code, the mobile station reads a SYNC channel which identifies the particular PN offset. Mobile stations on power-up acquire their time reference on the basis of the PN offset of the first base station they make contact with. This time reference is delayed by the RF propagation delay between the base station and the mobile station. The mobile station reads in another channel (the paging channel for example) information necessary for it to know how to acquire the system. This will include how to register, system parameters, call processing information, and neighbour list etc. After a mobile station acquires its time reference and reads the necessary information from the paging channel, the mobile station sends an access channel message to the base station which is aligned with the mobile station""s time reference. This access channel message will be received by the base station after a round trip delay representative of the sum of the time for the PN code to travel from the base station to the mobile station and the time for the mobile station""s access channel message to return from the mobile station to the base station.
In current base stations, CSMs (cell site modems) are provided to interact with mobile stations. These CSMs search for the access channel message sent by the mobile station within a fixed access search window. It is common to interchangeably refer to window sizes in units of chip durations (a chip is the unit of code spreading for CDMA) or in terms of a round trip distance. For CDMA, one chip is approximately 0.8 xcexcs in duration, and the xe2x80x9cchip distancexe2x80x9d is approximately 244 meters. In some existing systems the access search window is 125 km (512 chips) meaning that the base station will only see signals sent from mobile stations within a radius of 62.5 km (or 125 km round trip). An existing chip produced by Qualcomm which is in widespread use has this constraint, and in fact every chip will have some such physical constraint. Due to the physical nature of the problem, there is a need to limit the access window size.
It would be advantageous to be able to use existing mobile stations in environments with cells having large radii for interoperability reasons, and in particular in an environment in which existing cell site modems can be employed.
It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages.
Embodiments of the invention provide methods and apparatuses for providing CDMA (code division multiple access) access availability to a cell area, and for providing traffic channel establishment.
In one broad aspect, a plurality of cell site modems are configured to look for access attempts generated in respective circular or annular shaped areas centered at the centre of the cell area, the areas collectively covering the cell area.
The cell area might for example have a total radius R, and each cell site modem may be capable of searching over a respective range of distances. Preferably, a first of the cell site modems is configured to look within a circle of radius W, and subsequent cell site modems are configured to look within respective annular shaped areas which are sequentially increasing in diameter. Preferably, the annular shaped areas are substantially non-overlapping, and are of width W except for an outer most annular region which is of whatever width is required to completely cover the cell area.
Preferably, each cell site modem has a local clock and the local clock within the first of the cell site modems is synchronized with a standard network time, and the local clock of each of the subsequent cell site modems is offset by a respective delay with respect to the standard network time which causes the cell site modem to search within its respective annular shaped area.
Each of the cell site modems may be configured to look for an access attempt coded with an identical long code mask. Alternatively, some of the cell site modems may be configured to look for an access attempt coded with respective different long codes mask.
A cell site modem is preferably also made available as a traffic cell site modem and is operable to have its traffic acquisition search window centered at a location of an access attempt. Alternatively, a plurality of traffic cell site modems may be provided, each having dedicated circular or annular coverage areas within the cell. The traffic cell site modems also preferably have a local clock which may be synchronized, either to a standard network time, or to a respective delay which causes it to search in its respective area.
In another broad aspect, the invention provides an apparatus for providing CDMA (code division multiple access) traffic availability to a cell area. The apparatus has at least one cell site modem operable to look for traffic generated in respective configurable circular or annular shaped areas centered at the centre of the cell area. Alternatively, the areas covered by the cell site modems for traffic might be fixed and predetermined such that they permanently collectively cover the cell area.
The search windows of the traffic cell site modems may be controlled by adjustment of a delay to their local clocks, as was the case for the access cell site modems.
Another embodiment of the invention provides methods and systems for handing off between circular and annular shaped traffic coverage areas. One such method involves establishing an initial traffic acquisition search window in a cell site modem such that a mobile station""s energy is substantially at the centre of the search window, monitoring the position of a mobile station""s energy within the initial traffic acquisition search window, and when the mobile station""s energy becomes near an edge of the traffic acquisition search window, resynchronizing the cell site modem by positioning the search window so that the mobile station""s energy again falls substantially in the centre of the range available to the traffic acquisition search window thereby performing a radial handoff.
Monitoring the position of a mobile station within the initial traffic acquisition search window is preferably done by monitoring a round trip delay between the cell site modem and the mobile station.
Resynchronizing the cell site modem by positioning the traffic acquisition search window so that the mobile station""s energy again falls substantially in the centre of the range available to the traffic search window is preferably done by resynchronizing the cell cite modem with a timing offset selected to appropriately relocate the search window.
Preferably, such a radial handoff is triggered when the mobile station""s round trip delay indicates the mobile station""s energy is within a predetermined distance from the edge of the traffic acquisition search window.
Another embodiment of the invention provides a method of providing traffic coverage to a large cell in which permanent traffic rings within the large cell, and dedicated traffic channel CSMs are assigned to each of the traffic rings. When a mobile station moves from one ring to another, a handoff between the respective cell site modems is performed. A base station adapted to implement such a method is provided which consists of a plurality of cell site modems, with at least one cell site modem dedicated to provide traffic coverage to each of a plurality of permanent traffic rings defined within the large cell, a round trip delay tracker for tracking a round trip delay between the base station and a mobile terminal and determining when a mobile terminal is moving out of a previous traffic ring into a new traffic ring, and a radial handoff controller for switching the mobile terminal from a cell site modem in its previous ring to one in its new traffic ring.